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We report here on the occurrence of an interesting mantle-derived ultramafic xenolith entrained in an Eocene (c. 55 Ma) lamprophyre dyke from the Dongargaon area of the Chhotaudepur alkaline subprovince located within the Narmada Rift Zone, NW India. The mineralogy of the xenolith comprises olivine, clinopyroxene and mica (phlogopite), with the latter occurring essentially as rims around the clinopyroxene. Inclusions of apatite, interstitial sulphide (pyrite) and micron-scale exsolved spinel are widespread. Olivine is forsteritic (Fo85.34), displays little compositional variation and overlaps with that reported from worldwide mantle peridotite xenoliths. Clinopyroxene is a diopside with a compositional range of Wo48.36 En43.83, Fs6.53 and Ac1.27, and is conspicuous by its high CaO (up to 24.4 wt%) and TiO2 (up to 1.6 wt%) content. Clinopyroxene is also compositionally similar to that reported from ‘enriched’ (metasomatized) peridotite xenoliths rather than those that occur in the ‘normal’ (depleted) peridotitic xenoliths. Phlogopites have a high concentration of fluorine (up to 1 wt%), whereas the apatites show an anomalous enrichment of F (up to 5 wt%), as well as enrichment in Sr (SrO up to 1.9 wt%). Our study provides the first direct petrographical evidence for the modal metasomatism in the post-Deccan subcontinental lithospheric mantle (SCLM) from this domain. From the textural and mineralogical assemblage of the xenolith, we infer that a possible olivine + garnet + orthopyroxene assemblage, in the presence of a metasomatic fluid, has given rise to clinopyroxene + phlogopite + spinel. The paragenesis of apatite essentially as inclusions suggests that it to be the earliest crystallized phase during the metasomatic event. Geothermobarometry of the clinopyroxene in the xenolith reveal temperatures of approximately 1200°C and pressures of approximately 12 kb, which are comparable with such data reported from other Deccan-related xenoliths. Preservation of phlogopite and apatite in the ultramafic xenolith imply that some of the readily fusible metasomatized portions in this domain escaped wholesale melting during the eruption of the Deccan Traps, possibly due to the variable thickness of the underlying SCLM.

Existing geochronological information on Deccan indicates prolonged (started at 68.5 Ma) alkaline magmatism related to the Réunion mantle plume based on the 40Ar/39Ar ages from Sarnu-Dandali and Mundwara alkaline complexes. We studied in detail an alkaline lamprophyre, from the Sarnu-Dandali complex, rich in groundmass (magmatic) as well as xenocrystic phlogopites and clinopyroxenes. 40Ar/39Ar age determinations of the phlogopites from this lamprophyre, reveal two distinct ages of 65.44±1.5 Ma and 68.17±1Ma. However, paleomagnetic results show a VGP at 32.31 N and 298.52 E concordant with that of the Deccan Super Pole at 65.5 Ma and support the younger eruption age at ca. 65.44±1.5Ma. Analyzed phlogopites lack any signs of retention of excess radiogenic Ar and yield similar inverse isochron ages, which suggests that the older age of ca. 68.17±1Ma belongs to the crystallization of xenocrystic phlogopite during mantle metasomatism. Trace element compositions support derivation of lamprophyre magma from an OIB- type enriched (metasomatized) mantle source with an involvement of phlogopite.This finding suggests that the pre-Deccan ages of ca. 68-69 Ma reported previously, may reflect the timing of metasomatism of the subcratonic lithospheric mantle during the separation of Greater-Seychelles from India at ca. ∼68.5 Ma. The absence of pre-Deccan alkaline rocks therefore indicates the short-duration (occurred between 67-65 Ma) of alkaline as well as small-volume, volatile-rich magmatism directly related to the Réunion (Deccan) plume.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5490881

The Early to Late Cretaceous Mundwara alkaline complex (comprising the Musala, Mer and Toa plugs) displays a broad spectrum of alkaline rocks closely associated in space and time with the Deccan Large Igneous Province (DLIP) in NW India. Petrology and Nd-Sr isotopic data on two youngest and altogether compositionally different lamprophyre dykes of the Mundwara alkaline complex are presented in this paper to understand their petrogenesis and also to constrain the magmatic processes responsible for generation of the rock spectrum in the complex (pyroxenite, picrite ankaramite, carbonatite, shonkinite, olivine gabbro, feldspathoidal and foid-free syenite). The two lamprophyre dykes occurring in the Mer and the Musala hills are referred to as basaltic camptonite I and camptonite II, respectively. The basaltic camptonite-I is highly porphyritic and contains olivine, clinopyroxene and magnetite macrocrysts embedded within the groundmass of microphenocyrsts composed of clinopyroxene, phlogopite, magnetite and feldspar. Whereas camptonite-II, with more or less similar texture, contains amphibole, biotite, magnetite and clinopyroxene within the microphenocrystic groundmass of amphibole, biotite, apatite and feldspar. Pyroxenes are chemically zoned and display corrosion of the cores revealing that they are antecrysts developed during early stages of magma evolution and later on inherited by more evolved magmas. Mineral chemistry and trace element composition of the lamprophyres reveal that fractional crystallisation was a dominant process. Early segregation of olivine + Cr-rich clinopyroxene + Cr-spinel from a primary hydrous alkali basalt within a magmatic plumbing system is inferred which led to the generation of basaltic camptonitic magma (M1) forming the Mer hill lamprophyre. Subsequently, progressive fractionation of pyroxene and Fe-Ti oxides from the basaltic camptonitic (M1) magma generated camptonitic (M2) magma forming the Musala hill lamprophyre. Both lamprophyre dykes on the Sr-Nd isotopic array reflect plume type asthenospheric derivation which largely corresponds to the Réunion plume and other alkaline rocks of the Deccan LIP. Our study brings out a complex sequence of processes such as crystal fractionation, accumulation and corrosion in the magmatic plumbing system involved in the generation of the Mundwara alkaline complex.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5277073